I also had a take or two at a "Spacewar! for One". Neither made a dent in the universe. :-)

[1] Outer Orbit (closer to Spacewar! and its orbital mechanics): https://www.masswerk.at/outerorbit/

[2] Gravi-o-roids! (featuring multiple gravity attractors and probably the better game): https://www.masswerk.at/gravioroids/